Infra-red imaging devices typically employ an array of pyroelectric elements to generate the pixels of the infra-red image. Each pyroelectric element is a small capacitor whose capacitance changes with temperature. When an image is focused on the array, any given capacitor in the array is heated at a rate which depends on the intensity of the infra-red radiation in the image at the location sensed by that capacitor. The rate of increase in the temperature of each element can, in principle, be sensed by measuring the current flowing into or out of the capacitor in question. Unfortunately, the currents in question are quite small; hence, some form of signal averaging is typically used to improve the signal-to-noise ratio.
The signal averaging is typically performed by using an electro-optical or mechanical chopper to modulate the image at a predetermined frequency. The current detectors are then phase-locked to the modulation frequency to provide the improved signal-to-noise ratio. The phase-locked chopping schemes have been found to be expensive and unreliable in practice.
In addition, each capacitor in the sensing array must be individually addressed. Hence, the readout circuitry must include an isolation transistor for each capacitor. These transistors substantially increase the cost of a sensor array.
In addition, the isolation transistors reduce the sensitivity of the detectors. The isolation transistors introduce shot noise into the charge measurements. This noise source further aggravates the low signal-to-noise ratios inherent in these devices. The charges involved are very small; hence, the isolation transistors must be run in their non-linear region. In this region, the variation between isolation transistors becomes significant. As a result, prior art devices must be compensated for the variation in sensitivity from sensor to sensor that results from the variations in the isolation transistors. This compensation is normally performed by storing a calibration value for each pyroelectric element in the array and subtracting the calibration value from the measured value. This calibration procedure substantially increases the cost of the infra-red imaging device.
Broadly, it is the object of the present invention to provide an improved infrared imaging device.
It is a further object of the present invention to provide an infra-red imaging array in which the pyroelectric elements do not require isolation transistors.
It is a still further object of the present invention to provide an infra-red imaging array which does not require chopping of the image to enhance the signal-to-noise ratio.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.